1. Field of the Invention
The present invention relates generally to a method for forming complementary field effect transistors and more specifically to an improved method for forming complementary metal oxide semiconductor (CMOS) devices using a doped insulative layer and an impurity containing atmosphere in a double diffusion self-aligned process.
2. Description of the Prior Art
Processes for fabricating complementary field effect transistors have used doped glass or oxide as sources of impurities in a single diffusion step to form the self-aligned complementary structures. These processes generally include depositing the doped oxide or glass, performing a masking operation to remove a portion of the first doped oxide, applying a second layer of doped oxide of opposite impurity concentration and simultaneously diffusing the impurities from the doped oxide layers to the substrate. Sometimes an additional step placing a protective layer over the doped oxide layers to prevent out diffusion is included.
Other processes of forming simultaneously P and N type areas in a substrate have applied a glass or silicon dioxide doped region, delineated the region, and diffused from the doped layer simultaneously with vapor deposition and diffusion of impurities of opposite conductivity types. As with the previous process, the diffusion of the impurities into the substrate of both impurity types are performed simultaneously. In both of the two mentioned processes, since this diffusion of the impurities of opposite conductivity type are performed simultaneously, the choice of impurities are limited to dopants which have the same coefficient of diffusion. Thus the choice of impurities are restricted to boron and phosphorus.
Similarly, by simultaneous diffusion of dopants with opposite conductivity type, some protective measure must be taken to prevent the interaction of the dopants. As mentioned previously, additional method steps including a protective coating over the glass or silicon dioxide doped layer must be provided. Each additional process step, or movement of the composite structure between steps, increases the time and cost of production. Simultaneous diffusion also prevents individual control of the N-type and P-type region depths.